In recent years, there has been an increasing demand for more sophisticated electronic apparatuses or electronic apparatuses having more functions. In order for electronic apparatuses to be more sophisticated or have more functions, it is necessary to increase the size of components to be mounted in the electronic apparatuses. Further, it is necessary for the components of the electronic apparatuses to have terminals including I/O (input/output) terminals provided on their bottom surfaces like packages such as BGA (Ball Grid Array), LGA (Land Grid Array), BCC (Bump Chip Carrier), and QFN (Quad Flat Non-leaded package). That is, components having large bottom surfaces have been mounted on a board by being joined and electrically connected to the board at their bottom surfaces. In such joining, soldering is employed.
With respect to the material quality of solder used in soldering electronic apparatus components to a board, leaded solder has been replaced with lead-free solder in response to demands for addressing environmental issues. The melting point of lead-free solder is commonly higher than the melting point of leaded solder. For example, the melting point of 60/40 (wt %) Sn/Pb leaded solder is 183° C., and the melting point of 96.5/3/0.5 (wt %) Sn/Ag/Cu lead-free solder is 217° C. Accordingly, replacing leaded solder with lead-free solder causes an increase in soldering temperature.
In the case of soldering a component with terminals provided on its bottom surface to a board using such lead-free solder, voids are generated in the solder when the solder melts. The reaction that generates voids in lead-free solder is as follows:SnO+2C17H35COOH→(C17H35COO)2Sn+H2O.  (1)
Here, if the soldering temperature is high, tin (Sn) in the lead-free solder is oxidized into tin oxide (SnO), and the tin oxide reacts with a carboxylic acid contained in a flux to generate water (H2O) as illustrated in Eq. (1). The water, which is generated as vapor, causes voids to be generated in the solder.
Further, (C17H35COO)2Sn, which is a reaction product of the tin (Sn) and the carboxylic acid, reacts with the copper (Cu) of interconnects to result in solder joints as follows:(C17H35COO)2Sn+Cu→(C17H35COO)2Cu+Sn.  (2)
Here, the voids generated by the reaction illustrated in Eq. (1) reside between the bottom surface of the component and the board. The residence of the voids tilts the component, causes poor joining, or generates space in the solder, thus decreasing the reliability of soldering (solder joints). In particular, in the case of using lead-free solder, tin (Sn) in the lead-free solder is more likely to be oxidized because of higher soldering temperatures. This increases tin oxide (SnO) to accelerate the reaction illustrated in Eq. (1), so that generation of water (H2O) is accelerated to increase the amount of void generation.
Several methods have been proposed as joining methods that discharge voids generated in such soldering outside the solder.
For example, Japanese Laid-open Patent Publication No. 2006-165402 describes a method where a component and a board are placed and heated on a hot plate, and voids are discharged outside solder by tilting the board while observing the voids using X-rays.
Further, Japanese Laid-open Patent Publication No. 2001-58259 describes a method where voids are discharged outside solder by heating a board in a vacuum filled with hydrogen, which is a reducing gas.